Given a string, find the length of the longest substring T that contains at most k distinct characters.

For example, Given s = “eceba” and k = 2,

T is “ece” which its length is 3.

这道题从最基本的Longest Substring with At Most 2 Distinct Characters开始，基本思路是一样的，这类题必须按照sliding windows的套路解题。一开始维持一个左指针start, 新建一个hashMap来存放每个字母出现的次数。循环遍历右指针i, 往hashMap里更新distinct characters. 当hashMap的size() > k 的时候，我们开始通过start指针从左边删字符, 直到hash.size() <= k. 注意这里要先在hashMap里减少字符个数，当字符个数还剩1时继续减才能删除。

class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        if (s == null || s.length() == 0){
            return 0;
        }
        if (s.length() < k){
            return s.length();
        }
        int start = 0;
        int maxLen = k;
        HashMap<Character, Integer> hash = new HashMap<>();
        for (int i = 0; i < s.length(); i++){
            char ch = s.charAt(i);
            if (!hash.containsKey(ch)){
                hash.put(ch, 1);
            } else {
                hash.put(ch, hash.get(ch) + 1);
            }
            maxLen = Math.max(maxLen, i - start);
            while (hash.size() > 2){
                char cha = s.charAt(start);
                if (hash.get(cha) == 1){
                    hash.remove(cha);
                } else if (hash.get(cha) > 1){
                    hash.put(cha, hash.get(cha) - 1);
                }
                start++;
            }
        }
        maxLen = Math.max(maxLen, s.length() - start);
        return maxLen;
    }
}